Magnetoresistive element and magnetic memory

ABSTRACT

A magnetoresistive element according to an embodiment includes: a first to third ferromagnetic layers, and a first nonmagnetic layer, the first and second ferromagnetic layers each having an axis of easy magnetization in a direction perpendicular to a film plane, the third ferromagnetic layer including a plurality of ferromagnetic oscillators generating rotating magnetic fields of different oscillation frequencies from one another. Spin-polarized electrons are injected into the first ferromagnetic layer and induce precession movements in the plurality of ferromagnetic oscillators of the third ferromagnetic layer by flowing a current between the first and third ferromagnetic layers, the rotating magnetic fields are generated by the precession movements and are applied to the first ferromagnetic layer, and at least one of the rotating magnetic fields assists a magnetization switching in the first ferromagnetic layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 13/210,678filed Aug. 16, 2011, and is based upon and claims the benefit ofpriority from prior Japanese Patent Application No. 2011-68853 filed onMar. 25, 2011 in Japan, the entire contents of each of which areincorporated herein by reference.

FIELD

Embodiments described herein relate generally to a magnetoresistiveelement and magnetic memory.

BACKGROUND

It is known that a MTJ (Magnetic Tunnel Junction) element serving as amagnetoresistive element has a stacked structure as a basic structure,and shows a tunneling magnetoresistive (TMR) effect. The stackedstructure is formed by a first ferromagnetic layer, a tunnel barrierlayer, and a second ferromagnetic layer. Such MTJ elements are used in100 bits per square inch (Mbpsi) HDD heads and magnetic random accessmemories (MRAMs).

A MRAM characteristically stores information (“1”, “0”) depending onchanges in the relative angle of the magnetizations of magnetic layersincluded in each MTJ element, and therefore, is nonvolatile. Since amagnetization switching takes only several nanoseconds, high-speed datawriting and high-speed data reading can be performed. Therefore, MRAMsare expected to be the next-generation high-speed nonvolatile memories.If a technique called a spin torque transfer switching technique forcontrolling magnetization through spin-polarized current is used, thecurrent density can be made higher when the cell size of the MRAM ismade smaller. Accordingly, high-density, low-power-consumption MRAMsthat can readily invert the magnetization of a magnetic material can beformed.

Furthermore, in recent years, attention has been drawn to the theorythat a magnetoresistance ratio as high as 1000% can be achieved by usingMgO as the tunnel barrier layer. Since the MgO is crystallized,selective tunneling conduction of the electrons having a certainwavenumber from the ferromagnetic layer can be performed, and thoseelectrons can keep the wavenumber during that time. At this point, thespin polarizability has a large value in a certain crystallineorientation, and therefore, a giant magnetoresistive effect appears.Accordingly, an increase in the magnetoresistive effect of each MTJelement leads directly to a higher-density MRAM that consumes lesspower.

Meanwhile, to form high-density nonvolatile memories, high integrationof magnetoresistive elements is essential. However, the ferromagneticbodies forming magnetoresistive elements have thermal disturbanceresistance that is degraded with a decrease in element size. Therefore,how to improve the magnetic anisotropy and thermal disturbanceresistance of each ferromagnetic material is a critical issue.

To counter this problem, trial MRAMs using perpendicular-magnetizationMTJ elements in which ferromagnetic bodies have magnetization directionsperpendicular to the film plane have been made in recent years. In aperpendicular-magnetization MTJ element, a material having high magneticcrystalline anisotropy is normally used for ferromagnetic bodies. Such amaterial has a magnetization in a certain crystal direction, and themagnetic crystalline anisotropy of the material can be controlled bychanging the composition ratio between constituent elements and thecrystallinity of the constituent elements. Accordingly, themagnetization direction can be controlled by changing the direction ofcrystal growth. Also, since ferromagnetic bodies have high magneticcrystalline anisotropy, the aspect ratio of the element can be adjusted.Furthermore, having high thermal disturbance resistance, ferromagneticbodies are suited for integration. In view of those facts, to realize ahighly-integrated MRAM that consumes less power, it is critical tomanufacture perpendicular-magnetization MTJ elements that have a greatmagnetoresistive effect.

To further reduce the current required for performing writing in such aperpendicular-magnetization MTJ element, a magnetization oscillationlayer that generates a rotating magnetic field to assist magnetizationswitching at the time of writing is formed according to a known method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a magnetoresistive element accordingto a first embodiment;

FIGS. 2(a) through 2(c) are cross-sectional views of specific examplesof magnetization oscillation layers;

FIG. 3 is a diagram showing the resonant frequency characteristics ofthe magnetization oscillation layer;

FIG. 4 is a cross-sectional view of a magnetoresistive element accordingto a modification of the first embodiment;

FIG. 5 is a cross-sectional view of a magnetoresistive element accordingto a second embodiment;

FIG. 6 is a cross-sectional view of a magnetoresistive element accordingto a third embodiment;

FIG. 7A is a cross-sectional view of a magnetoresistive elementaccording to a first modification of the third embodiment;

FIG. 7B is a cross-sectional view of a magnetoresistive elementaccording to a second modification of the third embodiment;

FIG. 7C is a cross-sectional view of a magnetoresistive elementaccording to a third modification of the third embodiment;

FIG. 8 is a cross-sectional view of a magnetoresistive element accordingto a fourth embodiment;

FIG. 9 is a circuit diagram of a magnetic memory according to a fifthembodiment; and

FIG. 10 is a cross-sectional view of a memory cell of the magneticmemory according to the fifth embodiment.

DETAILED DESCRIPTION

A magnetoresistive element according to an embodiment includes: a firstferromagnetic layer having an axis of easy magnetization in a directionperpendicular to a film plane, a magnetization direction of the firstferromagnetic layer being changeable; a second ferromagnetic layerhaving an axis of easy magnetization in a direction perpendicular to thefilm plane, a magnetization direction of the second ferromagnetic layerbeing fixed; a first nonmagnetic layer formed between the firstferromagnetic layer and the second ferromagnetic layer; and a thirdferromagnetic layer formed on an opposite side from the firstnonmagnetic layer relative to the second ferromagnetic layer, andincluding a plurality of ferromagnetic oscillators generating rotatingmagnetic fields of different oscillation frequencies from one another,each of the oscillators having a magnetization parallel to the filmplane. Spin-polarized electrons are injected into the firstferromagnetic layer and induce precession movements in the plurality offerromagnetic oscillators of the third ferromagnetic layer by flowing acurrent between the first ferromagnetic layer and the thirdferromagnetic layer, the rotating magnetic fields are generated by theprecession movements and are applied to the first ferromagnetic layer,and at least one of the rotating magnetic fields assists a magnetizationswitching in the first ferromagnetic layer.

The following is a description of embodiments of the present invention,with reference to the accompanying drawings. In the followingdescription, components having the same functions and structures aredenoted by the same reference numerals, and description of them will berepeated only where necessary.

First Embodiment

FIG. 1 shows a magnetoresistive element 1 of a first embodiment. FIG. 1is a cross-sectional view of the magnetoresistive element 1 of the firstembodiment. The magnetoresistive element 1 of this embodiment is a MTJelement and has a structure that is formed by stacking a ferromagneticlayer 2, a nonmagnetic layer (also called a tunnel barrier layer) 4, aferromagnetic layer 6, and a magnetization oscillation layer 10 in thisorder. The ferromagnetic layers 2 and 6 each has an easy magnetizationdirection perpendicular to the film plane. That is, the MTJ element ofthis embodiment is a so-called perpendicular-magnetization MTJ elementin which the ferromagnetic layers 2 and 6 each has a magnetizationdirection perpendicular to the film plane. It should be noted that, inthis specification, “film plane” refers to the upper surface of eachferromagnetic layer. Also, “easy magnetization direction” is thedirection in which a macro-size ferromagnetic material has the lowestinternal energy when the spontaneous magnetization direction of themacro-size ferromagnetic material is the same as the direction in asituation where there is no external magnetic fields. On the other hand,“hard magnetization direction” is the direction in which a macro-sizeferromagnetic material has the largest internal energy when thespontaneous magnetization direction of the macro-size ferromagneticmaterial is the same as the direction in a situation where there are noexternal magnetic fields. When a write current flows into the MTJelement 1, the magnetization direction of one of the ferromagnetic layer2 and the ferromagnetic layer 6 does not change (or is fixed), and themagnetization direction of the other ferromagnetic layer is changeable.The ferromagnetic layer having a fixed magnetization direction is alsoreferred to as a reference layer, and the ferromagnetic layer having achangeable magnetization direction is also referred to as a storagelayer.

The magnetization oscillation layer 10 includes magnetic materials(oscillators) that have magnetization directions parallel to the filmplane. The frequencies of rotating magnetic fields generated from therespective magnetic materials (oscillation frequencies) differ from oneanother. When a current flowing in a direction perpendicular to the filmplane is applied to the magnetization oscillation layer 10, themagnetization precesses in the film plane or rotates, to generate arotating magnetic field. The frequency of the rotating magnetic field isdetermined by the magnetic parameters of the magnetization oscillationlayer 10 and the applied current density. The frequency of the rotatingmagnetic field is designed to be within the range including the resonantfrequency of the storage layer. When the frequency of the rotatingmagnetic field generated from the magnetization of the magnetizationoscillation layer 10 matches the resonant frequency observed when themagnetization of the ferromagnetic layer 2 serving as the storage layeris reversed, the magnetization switching in the ferromagnetic layer 2serving as the storage layer can be assisted, or the magnetizationswitching current in the ferromagnetic layer 2 can be reduced.

A first specific example of the magnetization oscillation layer 10 has agranular film structure in which magnetic fine grains 12 of differentsizes of several tends of nanometers are scattered in a nonmagneticmaterial 11 such as AlO, as shown in FIG. 2(a). The respective magneticfine grains 12 have magnetizations parallel to the film plane. Therespective magnetic fine grains 12 have a size (diameter) or magneticparameter distribution, and therefore, the magnetizations of therespective fine grains 12 rotate at different frequencies from oneanother. Accordingly, the magnetization oscillation layer 10 as anaggregate of the magnetic fine grains 12 oscillates at an oscillationfrequency having a distribution. That is, the effect to assistmagnetization switching in the storage layer can be achieved in a widecurrent range. If the magnetization oscillation layer 10 of the firstspecific example shown in FIG. 2(a) is used, a nonmagnetic layer ispreferably provided between the magnetization oscillation layer 10 andthe ferromagnetic layer 6.

A second specific example of the magnetization oscillation layer 10 maybe a structure in which magnetic materials 12 that penetrate through aninsulator 11 and have different sizes (diameters) are formed in theinsulator 11, as shown in FIG. 2(b). In this case, the effect to assistmagnetization switching in the storage layer can be achieved in a widecurrent range, as in the first specific example. If the magneticoscillation layer 10 of the second specific example shown in FIG. 2(b)is used, a nonmagnetic layer is preferably provided between themagnetization oscillation layer 10 and the ferromagnetic layer 6.

In a third specific example, the magnetization oscillation layer 10 hasa stacked structure formed with ferromagnetic films and a nonmagneticfilm. For example, a ferromagnetic film 10 a, a nonmagnetic film 10 b,and a ferromagnetic film 10 c are stacked in this order to form thestacked structure, as shown in FIG. 2(c). The respective ferromagneticfilms 10 a and 10 c have different oscillation frequencies from eachother, and the oscillation frequencies are set in a region near theresonant frequency of the storage layer. That is, in this case, theeffect to assist magnetization switching in the storage layer can alsobe achieved in a wide current range. If the magnetic oscillation layer10 of the third specific example shown in FIG. 2(c) is used, anonmagnetic layer is preferably provided between the magnetizationoscillation layer 10 and the ferromagnetic layer 6. In FIG. 2(c), twoferromagnetic films having a nonmagnetic film interposed in between areshown, but three or more ferromagnetic films may be provided.

Next, an operation to be performed by the magnetoresistive element 1 ofthis embodiment at the time of writing is described. A write currentflowing in a direction perpendicular to the film plane is appliedbetween the ferromagnetic layer 2 and the ferromagnetic layer 6. Itshould be noted that, in this description of the operation, theferromagnetic layer 2 serves as the storage layer, and the ferromagneticlayer 6 serves as the reference layer.

First, in a case where the magnetization direction of the ferromagneticlayer 2 and the magnetization direction of the ferromagnetic layer 6 areantiparallel (or the opposite directions from each other), a current isapplied from the ferromagnetic layer 2 to the magnetization oscillationlayer 10 via the nonmagnetic layer 4 and the ferromagnetic layer 6. Inthis case, electrons flow from the magnetization oscillation layer 10 tothe ferromagnetic layer 2 via the ferromagnetic layer 6 and thenonmagnetic layer 4. The electrons that are spin-polarized through theferromagnetic layer 6 flow into the ferromagnetic layer 2. Thespin-polarized electrons having spins in the same direction as themagnetization direction of the ferromagnetic layer 2 pass through theferromagnetic layer 2. However, the spin-polarized electrons havingspins in the opposite direction from the magnetization direction of theferromagnetic layer 2 apply a spin torque to the magnetization of theferromagnetic layer 2, so that the magnetization direction of theferromagnetic layer 2 becomes the same as the magnetization direction ofthe ferromagnetic layer 6. At this point, the magnetization oscillationlayer 10 generates a rotating magnetic field having a frequency within arange including the resonant frequency of the ferromagnetic layer 2serving as the storage layer, and a rotating magnetic field parallel tothe film plane is applied to the ferromagnetic layer 2. That is, amagnetization switching in the ferromagnetic layer 2 serving as thestorage layer is assisted. As a result of this, the magnetizationdirection of the ferromagnetic layer 2 is reversed, and becomes parallelto (or the same as) the magnetization direction of the ferromagneticlayer 6.

In a case where the magnetization direction of the ferromagnetic layer 2and the magnetization direction of the ferromagnetic layer 6 areparallel, a write current is applied from the magnetization oscillationlayer 10 to the ferromagnetic layer 2 via the ferromagnetic layer 6 andthe nonmagnetic layer 4. In this case, electrons flow from theferromagnetic layer 2 to the magnetization oscillation layer 10 via thenonmagnetic layer 4 and the ferromagnetic layer 6. As a result, theelectrons that are spin-polarized through the ferromagnetic layer 2 flowinto the ferromagnetic layer 6. The spin-polarized electrons havingspins in the same direction as the magnetization direction of theferromagnetic layer 6 pass through the ferromagnetic layer 6. However,the spin-polarized electrons having spins in the opposite direction fromthe magnetization direction of the ferromagnetic layer 6 are reflectedby the interface between the nonmagnetic layer 4 and the ferromagneticlayer 6, flow into the ferromagnetic layer 2 through the nonmagneticlayer 4, and apply a spin torque to the magnetization of theferromagnetic layer 2, so that the magnetization direction of theferromagnetic layer 2 becomes the opposite from the magnetizationdirection of the ferromagnetic layer 6. At this point, the magnetizationoscillation layer 10 generates a rotating magnetic field having afrequency within a range including the resonant frequency of theferromagnetic layer 2 serving as the storage layer, and a rotatingmagnetic field parallel to the film plane is applied to theferromagnetic layer 2. That is, a magnetization switching in theferromagnetic layer 2 serving as the storage layer is assisted. As aresult of this, the magnetization direction of the ferromagnetic layer 2is reversed, and becomes antiparallel to the magnetization direction ofthe ferromagnetic layer 6.

As shown in FIG. 3, in the first embodiment, the magnetizationoscillation layer 10 has the different oscillation frequencycharacteristics indicated by the solid lines in the range A includingthe resonant frequency f₀ of the ferromagnetic layer 2 serving as thestorage layer. Accordingly, the oscillation frequency characteristics ofthe magnetization oscillation layer 10 has characteristics combining thedifferent oscillation frequency characteristics indicated by the solidlines, or have the wide oscillation frequency band indicated by thebroken line. As the magnetization oscillation layer 10 has such a wideoscillation frequency band and has an oscillation frequency proportionalto the current density as described later, the magnetization oscillationlayer 10 resonates with a wide range of current values, and the writecurrent can be reduced. If the frequency of the rotating magnetic fieldgenerated from the magnetization oscillation layer 10 is in the range of2.5 GHz to 6.0 GHz, the preferred range A including the resonantfrequency f₀ of the ferromagnetic layer 2 serving as the storage layeris the range of 0.62f₀ to 1.50f₀ with respect to the resonant frequencyf₀ of the ferromagnetic layer 2 serving as the storage layer. In thisrange (0.62f₀ to 1.50f₀), an assisting effect for the storage layer isachieved, as disclosed in the specification of another application (JP-A2009-231753(KOKAI)) filed by the applicant. Therefore, the magnetizationoscillation layer 10 preferably generates an oscillation frequency inthe range of 0.62f₀ to 1.50f₀ with respect to the resonant frequency f₀of the ferromagnetic layer 2 serving as the storage layer.

As described above, according to the first embodiment, the write currentrange can be made wider, and the write current can be made smaller.

Even if the resonant frequency of the ferromagnetic layer 2 serving asthe storage layer varies or fluctuates, or even if the oscillationfrequency of the magnetization oscillation layer 10 varies orfluctuates, the natural frequency of the storage layer can fall withinthe oscillation frequency band of the magnetization oscillation layer10. As a result, highly-efficient spin-injection writing using resonancecan be stably performed.

(Modification)

FIG. 4 shows a magnetoresistive element according to a modification ofthe first embodiment. The magnetoresistive element 1A of thismodification is the same as the magnetoresistive element of the firstembodiment shown in FIG. 1, except that the magnetization oscillationlayer 10 is provided on the opposite side of the ferromagnetic layer 2serving as the storage layer from the nonmagnetic layer 4.

In this modification, the write current range can be made wider, and thewrite current can be made smaller, as in the first embodiment. Also,highly-efficient spin-injection writing can be stably performed.

Second Embodiment

FIG. 5 shows a magnetoresistive element according to a secondembodiment. The magnetoresistive element 1B of the second embodiment isthe same as the magnetoresistive element 1 of the first embodiment shownin FIG. 1, except that a nonmagnetic layer 8 and a ferromagnetic layer 9are provided between the ferromagnetic layer 6 and the magnetizationoscillation layer 10. The ferromagnetic layer 9 is also called a biaslayer, and absorbs and adjusts a shift in the switching current for themagnetization reversal of the ferromagnetic layer 2 serving as thestorage layer due to a leak magnetic field generated from theferromagnetic layer 6 serving as the reference layer. The magnetizationdirection of the ferromagnetic layer 9 is preferably antiparallel to (orthe opposite of) the magnetization direction of the ferromagnetic layer6. Also, the ferromagnetic layer 9 may be antiferromagnetically coupledto the ferromagnetic layer 6 via the nonmagnetic layer 8 (SyntheticAnti-Ferromagnetic (SAF) coupling).

The nonmagnetic layer 8 preferably has the adequate thermal stability sothat the ferromagnetic layer 6 and the ferromagnetic layer 9 do not mixwith each other in a heating process. The nonmagnetic layer 8 alsopreferably has a function to control the crystalline orientation whenthe ferromagnetic layer 9 is formed. Further, if the nonmagnetic layer 8is thick, the distance between the ferromagnetic layer 9 and theferromagnetic layer 2 serving as the storage layer is long, and theshift adjusting magnetic field to be applied from the ferromagneticlayer 9 to the storage layer is small. Therefore, the film thickness ofthe nonmagnetic layer 8 is preferably 5 nm or smaller. The ferromagneticlayer 9 is made of a ferromagnetic material that has an easy axis ofmagnetization in a direction perpendicular to the film plane. Since theferromagnetic layer 9 is further away from the ferromagnetic layer 2serving as the storage layer than the ferromagnetic layer 6 serving asthe reference layer is, the film thickness or the saturationmagnetization M_(S) of the ferromagnetic layer 9 need to be greater thanthose of the reference layer so that the ferromagnetic layer 9 adjuststhe leak magnetic field to be applied to the storage layer. That is,according to the results of the study made by the inventors, where t₂and M_(S2) represent the film thickness and saturation magnetization ofthe ferromagnetic layer 6 serving as the reference layer, and t₄ andM_(S4) represent the film thickness and saturation magnetization of theferromagnetic layer 9, the following relational expression should besatisfied:M _(S2) ×t ₂ <M _(S4) ×t ₄

In the second embodiment, the write current range can be made wider, andthe write current can be made smaller, as in the first embodiment. Also,highly-efficient spin-injection writing can be stably performed.

It should be noted that each spin injection layer 7 used in the laterdescribed third embodiment and its modifications and in the laterdescribed fourth embodiment can be made to function as a bias layer byappropriately selecting the film thickness and saturation magnetization.

Third Embodiment

FIG. 6 shows a magnetoresistive element according to a third embodiment.The magnetoresistive element 1C of the third embodiment is the same asthe magnetoresistive element 1 of the first embodiment shown in FIG. 1,except that a spin injection layer 7 having a magnetization directionantiparallel to the magnetization of the ferromagnetic layer 6 isprovided between the ferromagnetic layer 6 and the magnetizationoscillation layer 10, and the magnetization oscillation layer 10 has thegranular film structure shown in FIG. 2(a). That is, the magnetizationoscillation layer 10 has ferromagnetic grains 12 surrounded by anonmagnetic matrix 11. The ferromagnetic grains 12 are magneticallyindependent of one another. When a current flowing in a directionsubstantially perpendicular to the film plane is applied, spin-polarizedelectrons are injected from the spin injection layer 7 into themagnetization oscillation layer 10. As a result, the respectiveferromagnetic grains 12 oscillate to generate a high-frequency rotatingmagnetic field. In this manner, highly-efficient spin-injection writingcan be performed. The oscillation frequencies of the ferromagneticgrains differ from one another, and there exists at least onecombination of two ferromagnetic grains having oscillation frequenciesthat differ from each other by 20 to 50%. The size of themagnetoresistive element 1B is approximately 30 nm in diameter, and thenumber of ferromagnetic grains 12 contained in the magnetoresistiveelement 1B is 2 to 10. The thicknesses of the ferromagnetic grains 12vary from 1 nm to 3 nm, the diameters vary from 3 nm to 12 nm, and thelargest volume is approximately 50 times greater than the smallestvolume.

Each oscillation frequency fi at the time of spin injection from thespin injection layer 7 into the magnetization oscillation layer 10 isexpressed by the following mathematical formula obtained by solving theLandau-Lifshitz-Gilbert (LLG) equation:

$\begin{matrix}{f_{i} = {\frac{\gamma}{2{\pi\alpha}}( \frac{\overset{\_}{h}}{2e} )\frac{g(\theta)}{M_{s}t}J}} & (1)\end{matrix}$

However, if the nonmagnetic matrix 11 of the magnetization oscillationlayer 10 is a metal, the following relationship is satisfied:

$\begin{matrix}{{g(\theta)} = \lbrack \frac{{- 4} + {( {1 + P} )^{3}( {3 + {\cos\;\theta}} )}}{4P^{3/2}} \rbrack^{- 1}} & (2)\end{matrix}$

If the nonmagnetic matrix 11 of the magnetization oscillation layer 10is an insulator, the following relationship is satisfied:

$\begin{matrix}{{g(\theta)} = {\frac{1}{2}\frac{P}{1 + {P^{2}\cos\;\theta}}}} & (3)\end{matrix}$

Here, γ represents the gyromagnetic constant, α represents the dampingconstant, P represents the polarization, M_(S) represents the saturationmagnetization, t represents the thickness of the oscillator grains 12 ofthe magnetization oscillation layer 10, J represents the density ofcurrent flowing in the magnetization oscillation layer 10, h-barrepresents the Dirac constant obtained by dividing the Planck constant hby 2π, and e represents the elementary charge. Further, θ represents therelative angle between the magnetization of the magnetizationoscillation layer 10 and the magnetization of the spin injection layer 7under oscillating conditions.

Therefore, if the damping constants, the saturation magnetizations, thevolumes, the spin injection efficiencies, and the injection currents ofthe oscillators of the magnetization oscillation layer 10 vary, theoscillation frequencies of the oscillators also vary.

According to the mathematical formula (1), the oscillation frequency fiis proportional to the product of g(θ) and the current density J, and isinversely proportional to the film thickness t of the magnetizationoscillation layer 10. Therefore, when a uniform current flows in themagnetization oscillation layer 10, the oscillation frequency fi doesnot depend on the area even if the largest area is ten times larger thanthe smallest area, and depends on the thickness t of the ferromagneticgrain 12 and g(θ). Furthermore, in a case where the thicknesses t of allthe ferromagnetic grains 12 are uniform, the oscillation frequency fidepends only on g(θ), and ultimately, the oscillation frequency fi isdetermined by θ, which is determined by the demagnetizing field inaccordance with the shape of the ferromagnetic grain 12.

In this embodiment, the nonmagnetic matrix 11 may be an insulatingmaterial such as AlOx or MgO, or a metal material such as Cu, Ag, or Au.In a case where an insulating material is used, oscillation is caused byspin injection using a TMR effect. In a case where a metal material isused, oscillation is caused by spin injection using a GMR effect.

As described above, in the third embodiment, the write current range canbe made wider, and the write current can be made smaller, as in thefirst embodiment. Also, even if the resonant frequency of theferromagnetic layer 2 serving as the storage layer varies or fluctuates,or even if the oscillation frequency of the magnetization oscillationlayer 10 varies or fluctuates, the resonant frequency of the storagelayer can fall within the oscillation frequency band of themagnetization oscillation layer 10. As a result, highly-efficientspin-injection writing using resonance can be stably performed.

(First Modification)

FIG. 7A shows a magnetoresistive element according to a firstmodification of the third embodiment. The magnetoresistive element 1D ofthe first modification is the same as the magnetoresistive element 1C ofthe third embodiment shown in FIG. 6, except that the spin injectionlayer 7 and the magnetization oscillation layer 10 are provided on theopposite side of the ferromagnetic layer 2 serving as the storage layerfrom the nonmagnetic layer 4. In the first modification, the spininjection layer 7 is interposed between the ferromagnetic layer 2 andthe magnetization oscillation layer 10.

(Second Modification)

FIG. 7B shows a magnetoresistive element according to a secondmodification of the third embodiment. The magnetoresistive element 1D₁of the second modification is the same as the magnetoresistive element1D of the first modification shown in FIG. 7A, except that the positionsof the spin injection layer 7 and the magnetization oscillation layer 10are reversed. That is, the spin injection layer 7 is provided on theopposite side of the magnetization oscillation layer 10 from theferromagnetic layer 2. In the second modification, a nonmagnetic layer14 is preferably inserted between the ferromagnetic layer 2 and themagnetization oscillation layer 10 so that spin-polarized electrons donot move between the ferromagnetic layer 2 and the magnetizationoscillation layer 10. The nonmagnetic layer 14 should be made of amaterial having a small spin diffusion length. In the secondmodification, the magnetization oscillation layer 10 is located close tothe ferromagnetic layer 2 serving as the storage layer, and accordingly,the magnetization oscillation layer 10 can more effectively assistmagnetization switching in the ferromagnetic layer 2.

Also, in the second modification, the bias layer 9 described in thesecond embodiment may be provided on the opposite side of theferromagnetic layer 6 from the nonmagnetic layer 4. In such a case, thenonmagnetic layer 8 is preferably interposed between the ferromagneticlayer 6 and the bias layer 9 as in the second embodiment. By providingthe bias layer 9, a shift in the switching current for the magnetizationreversal of the ferromagnetic layer 2 serving as the storage layer canbe absorbed and adjusted.

(Third Modification)

FIG. 7C shows a magnetoresistive element according to a thirdmodification of the third embodiment. The magnetoresistive element 1D₂of the third modification is the same as the magnetoresistive element 1Cof the third embodiment shown in FIG. 6, except that the positions ofthe spin injection layer 7 and the magnetization oscillation layer 10are reversed. That is, the spin injection layer 7 is provided on theopposite side of the magnetization oscillation layer 10 from theferromagnetic layer 6. In the third modification, the nonmagnetic layer14 is preferably inserted between the ferromagnetic layer 6 and themagnetization oscillation layer 10 as in the second modification.

In the above described first through third modifications, the writecurrent range can be made wider, and the write current can be madesmaller as in the third embodiment. Also, highly-efficientspin-injection writing can be stably performed.

Fourth Embodiment

FIG. 8 shows a magnetoresistive element of a fourth embodiment. Themagnetoresistive element 1E of the fourth embodiment is the same as themagnetoresistive element 1C of the third embodiment shown in FIG. 6,except that the magnetization oscillation layer 10 is replaced with amagnetization oscillation layer 10A having a stacked structure in whichferromagnetic films and nonmagnetic films are stacked. The magnetizationoscillation layer 10A has a structure formed by stacking a nonmagneticfilm 10Aa, a ferromagnetic film 10Ab, a nonmagnetic film 10Ac, aferromagnetic film 10Ad, a nonmagnetic film 10Ae, and a ferromagneticfilm 10Af in this order on the spin injection layer 7. Thoseferromagnetic films 10Ab, 10Ad, and 10Af are magnetically independent ofone another, and are not coupled to one another.

The ferromagnetic films 10Ab, 10Ad, and 10Af differ in film thickness ormaterial. For example, three ferromagnetic films (such as CoFe films)having film thicknesses of 2 nm, 2.5 nm, and 3 nm are stacked, with 2-nmthick Cu films being interposed between the ferromagnetic films. Theratio between the maximum oscillation frequency and the minimumoscillation frequency is 1.5. To perform high-efficiency writing withresonance, oscillation should be caused at a frequency of 0.62f₀ to1.50f₀ with respect to the resonant frequency f₀ of the storage layer.Therefore, if oscillation is caused at two frequencies of 4 GHz and 6GHz, a desired effect can be achieved in a wide f₀ range of 2.7 to 10GHz.

In the fourth embodiment, the write current range can be made wider, andthe write current can be made smaller, as in the third embodiment. Also,highly-efficient spin-injection writing can be stably performed.

In the following, the structures of respective layers in the MTJelements 1, 1A, 1B, 1C, 1D, 1D₁, 1D₂, and 1E of the first through fourthembodiments and their modifications are specifically described. Theferromagnetic layer 2, the ferromagnetic layer 6, the spin injectionlayer 7, the magnetization oscillation layer 10, and the nonmagneticlayer 4 are described in this order.

(Ferromagnetic Layer 2)

The ferromagnetic layer 2 has an easy axis of magnetization in adirection perpendicular to the film plane. The material used as theferromagnetic layer 2 can be a metal that has (111) crystallineorientation of a face-centered cubic (FCC) structure or has (001)crystalline orientation of a hexagonal close-packed (HCP) structure, ora metal that can form an artificial lattice, for example. An example ofthe metal that has the (111) crystalline orientation of a FCC structureor the (001) crystalline orientation of a HCP structure is an alloycontaining at least one element selected from the first group consistingof Fe, Co, Ni, and Cu, and at least one element selected from the secondgroup consisting of Pt, Pd, Rh, and Au. Specifically, the metal is aferromagnetic alloy such as CoPd, CoPt, NiCo, or NiPt.

The artificial lattice used in the ferromagnetic layer 2 can be astructure in which one element of Fe, Co, and Ni or an alloy containingthe one element (a ferromagnetic film), and one element of Cr, Pt, Pd,Ir, Rh, Ru, Os, Re, Au, and Cu or an alloy containing the one element (anonmagnetic film) are alternately stacked. For example, the artificiallattice can be a Co/Pt artificial lattice, a Co/Pd artificial lattice, aCoCr/Pt artificial lattice, a Co/Ru artificial lattice, a Co/Osartificial lattice, a Co/Au artificial lattice, or a Ni/Cu artificiallattice. In each of the artificial lattices, the magnetic anisotropicenergy density and saturation magnetization can be controlled byadjusting the addition of an element to the ferromagnetic film and thefilm thickness ratio between the ferromagnetic film and the nonmagneticfilm.

(Ferromagnetic Layer 6)

The ferromagnetic layer 6 has an easy axis of magnetization in adirection perpendicular to the film plane. The material used as theferromagnetic layer 6 can be a metal that has (111) crystallineorientation of a face-centered cubic (FCC) structure or has (001)crystalline orientation of a hexagonal close-packed (HCP) structure, ora metal that can form an artificial lattice, for example. An example ofthe metal that has the (111) crystalline orientation of a FCC structureor the (001) crystalline orientation of a HCP structure is an alloycontaining at least one element selected from the first group consistingof Fe, Co, Ni, and Cu, and at least one element selected from the secondgroup consisting of Pt, Pd, Rh, and Au. Specifically, the metal is aferromagnetic alloy such as CoPd, CoPt, NiCo, or NiPt.

The artificial lattice used in the ferromagnetic layer 6 can be astructure in which one element of Fe, Co, and Ni or an alloy containingthe one element (a ferromagnetic film), and one element of Cr, Pt, Pd,Ir, Rh, Ru, Os, Re, Au, and Cu or an alloy containing the one element (anonmagnetic film) are alternately stacked. For example, the artificiallattice can be a Co/Pt artificial lattice, a Co/Pd artificial lattice, aCoCr/Pt artificial lattice, a Co/Ru artificial lattice, a Co/Osartificial lattice, a Co/Au artificial lattice, or a Ni/Cu artificiallattice. In each of the artificial lattices, the magnetic anisotropicenergy density and saturation magnetization can be controlled byadjusting the addition of an element to the ferromagnetic film and thefilm thickness ratio between the ferromagnetic film and the nonmagneticfilm.

Also, the material used as the ferromagnetic layer 6 can be an alloycontaining at least one element selected from the group consisting oftransition metals Fe, Co, and Ni, and at least one element selected fromthe group consisting of rare-earth metals Tb, Dy, and Gd. For example,the material can be TbFe, TbCo, TbFeCo, DyTbFeCo, or GdTbCo.Alternatively, the material can be a multilayer structure formed byalternately stacking those alloys. Specifically, such a multilayerstructure can be a multilayer film such as a TbFe/Co film, a TbCo/Fefilm, a TbFeCo/CoFe film, a DyFe/Co film, a DyCo/Fe film, or aDyFeCo/CoFe film. In each of those alloys, the magnetic anisotropyenergy density and saturation magnetization can be controlled byadjusting the film thickness ratio and the composition.

The material used as the ferromagnetic layer 6 can also be an alloycontaining at least one element selected from the first group consistingof Fe, Co, Ni, and Cu, and at least one element selected from the secondgroup consisting of Pt, Pd, Rh, and Au. Specifically, the material canbe a ferromagnetic alloy such as FeRh, FePt, FePd, or CoPt.

(Spin Injection Layer 7)

As the spin injection layer 7, the same material as the ferromagneticlayer 6 can be used.

(Magnetization Oscillation Layer 10)

The magnetization oscillation layer 10 has nanometer-size metalferromagnetic granules scattered in a nonmagnetic matrix. In a casewhere the nonmagnetic matrix is an insulator, the material of thenonmagnetic matrix is an oxide, a nitride, or a carbide containing atleast one element selected from Al, Si, Ti, Mg, Ta, Zn, Fe, Co, and Ni,and the ferromagnetic fine grains contain at least one element selectedfrom Fe, Co, and Ni. In a case where the nonmagnetic matrix is aconductor, the material of the nonmagnetic matrix can be a nonmagneticmetal material such as Cu, Ag, or Au. In this case, the ferromagneticfine grains also contain at least one element selected from Fe, Co, andNi.

In a case where the magnetization oscillation layer 10 has a stackedstructure formed by stacking ferromagnetic films and nonmagnetic films,the ferromagnetic films contain at least one element selected from Fe,Co, and Ni, and the nonmagnetic films contain at least one elementselected from Cu, Ag, Au, and Ru.

(Nonmagnetic Layer 4)

The nonmagnetic layer 4 is made of an insulating material, andtherefore, a tunnel barrier layer is used as the nonmagnetic layer 4.The tunnel barrier layer material can be an oxide having a maincomponent that is one element selected from the group consisting ofmagnesium (Mg), calcium (Ca), barium (Ba), aluminum (Al), beryllium(Be), strontium (Sr), and titanium (Ti). Specifically, the tunnelbarrier layer material can be MgO, CaO, BaO, AlO, BeO, SrO, or TiO. Thetunnel barrier layer can be a mixed crystalline material containing twoor more materials selected from the above-mentioned oxide group.Examples of mixed crystalline materials include MgAlO, MgTiO, and CaTiO.

The tunnel barrier layer can be either a crystalline material or anamorphous material. If the tunnel barrier layer is a crystalline layer,however, electron scattering can be restrained in the tunnel barrier,and the probability of selective tunneling conduction of electrons fromthe ferromagnetic layer while wavenumber is kept becomes higher.Accordingly, the magnetoresistance ratio can be made higher.

To increase the magnetoresistance ratio of the magnetoresistive element,an interfacial layer made of a material having a high spinpolarizability is preferably provided adjacent to the tunnel barrierlayer of the nonmagnetic layer 4 made of MgO. The interfacial layer ispreferably made of an alloy containing at least one metal selected fromthe group consisting of Fe and Co, for example. Also, to control thesaturation magnetization, at least one element selected from the groupconsisting of Ni, B, C, P, Ta, Ti, Mo, Si, W, Nb, Mn, and Ge can beadded as the interfacial layer. That is, the interfacial layer is analloy containing at least one element selected from the group consistingof Fe and Co, and at least one element selected from the groupconsisting of Ni, B, C, P, Ta, Ti, Mo, Si, W, Nb, Mn, and Ge. Forexample, the alloy can be CoFeB, but can also be CoFeSi, CoFeP, CoFeW,or CoFeNb. Those alloys have the same spin polarizability as that ofCoFeB. Alternatively, the interfacial layer can be a Heusler metal suchas Co₂FeSi, Co₂MnSi, or Co₂MnGe. A Heusler metal has a spinpolarizability equal to or higher than that of CoFeB, and therefore, issuited to be the interfacial layer. If an interfacial layer made ofCoFe, a nonmagnetic layer made of MgO, and an interfacial layer made ofCoFe are formed, for example, the epitaxial relationship of CoFe(001)/MgO (001)/CoFe (001) can be formed. In this case, the wavenumberselectivity of the tunneling electrons can be improved, and accordingly,a higher magnetoresistance ratio can be achieved.

Fifth Embodiment

The MTJ elements 1, 1A, 1B, 1C, 1D, and 1E of the first through fourthembodiments and their modifications can be applied to MRAMs. In thefollowing, for ease of description, an example case where the MTJelement 1 of the first embodiment is used is described.

Each memory element forming a MRAM includes a storage layer that has avariable (or reversible) magnetization (or spin) direction, a referencelayer that has an invariable (or fixed) magnetization direction, and anonmagnetic layer interposed between the storage layer and the referencelayer. Where “the magnetization direction of the reference layer isinvariable”, the magnetization direction of the reference layer does notchange when the magnetization reversing current to be used for switchingthe magnetization direction of the storage layer is applied to thereference layer. Since the two ferromagnetic layers each having an easyaxis of magnetization in a direction perpendicular to the film planeserve as the storage layer and the reference layer, a MRAM including MTJelements as memory elements can be formed.

Specifically, the two ferromagnetic layers are made to have a differencein coercive force from each other, so that the two ferromagnetic layerscan be used as the storage layer and the reference layer. Accordingly,in a MTJ element, a ferromagnetic layer having a large switching currentfor magnetization reversal is used as one ferromagnetic layer (thereference layer), and a ferromagnetic layer having a smaller switchingcurrent than the ferromagnetic layer serving as the reference layer isused as the other ferromagnetic layer (the storage layer). In thismanner, a MTJ element including a ferromagnetic layer with a variablemagnetization direction and a ferromagnetic layer with a fixedmagnetization direction can be realized.

FIG. 9 is a circuit diagram showing the structure of the MRAM accordingto the fourth embodiment. The MRAM of this embodiment includes memorycells arranged in a matrix fashion, and each of the memory cellsincludes the MTJ element 1. One end of each of the MTJ elements 1 iselectrically connected to a bit line BL. One end of each bit line BL iselectrically connected to a sense amplifier SA via an N-channel MOStransistor ST1 serving as a select switch. The sense amplifier SAcompares a read potential Vr and a reference potential Vref suppliedfrom a MTJ element 1, and outputs the result of the comparison as anoutput signal DATA. A resistor Rf electrically connected to the senseamplifier SA is a feedback resistor.

The other end of each bit line BL is electrically connected to the drainof a P-channel MOS transistor P1 and the drain of an N-channel MOStransistor N1 via an N-channel MOS transistor ST2 serving as a selectswitch. The source of the MOS transistor P1 is connected to a supplyterminal Vdd, and the source of the MOS transistor N1 is connected to aground terminal Vss.

The other end of each of the MTJ elements 1 is electrically connected toa lower electrode 29. Each of the lower electrodes 29 is electricallyconnected to a source line SL via an N-channel MOS transistor ST3serving as a select switch. It should be noted that the source line SLextends in a direction parallel to the bit lines BL.

The source line SL is electrically connected to the drain of a P-channelMOS transistor P2 and the drain of an N-channel MOS transistor N2 via anN-channel MOS transistor ST4 serving as a select switch. The source ofthe MOS transistor P2 is connected to the supply terminal Vdd, and thesource of the MOS transistor N2 is connected to the ground terminal Vss.The source line SL is also connected to the ground terminal Vss via anN-channel MOS transistor ST5 serving as a select switch.

The gate of each MOS transistor ST3 is electrically connected to a wordline WL. Each word line WL extends in a direction perpendicular to thedirection in which the bit lines BL extend.

Data writing into each MTJ element 1 is performed by a spin-injectionwriting technique. That is, the direction of the write current flowingin each MTJ element 1 is controlled by switching on and off the MOStransistors P1, P2, N1, and N2 with control signals A, B, C, and D, soas to realize data writing.

Data reading from each MTJ element 1 is performed by supplying a readcurrent to the MTJ element 1. The read current is set at a smaller valuethan the write current. Each MTJ element 1 has a resistance value thatvaries depending on whether the magnetization directions of thereference layer and the storage layer are parallel or antiparallel,because of a magnetoresistive effect. That is, the resistance value ofthe MTJ element 1 becomes the smallest when the magnetization directionsof the reference layer and the storage layer are parallel, and theresistance value of the MTJ element 1 becomes the largest when themagnetization directions of the reference layer and the storage layerare antiparallel. Such changes in resistance value are detected by thesense amplifier SA, to read the information recorded in the MTJ element1.

FIG. 10 is a cross-sectional view of one of the above described memorycells. An element isolation insulating layer 22 having a Shallow TrenchIsolation (STI) structure is formed in a P-type semiconductor substrate21. The N-channel MOS transistor ST3 as a select switch is formed in theelement region (active region) surrounded by the element isolationinsulating layer 22. The MOS transistor ST3 includes diffusion regions23 and 24 serving as source/drain regions, a gate insulating film 25formed on the channel region between the diffusion regions 23 and 24,and a gate electrode 26 formed on the gate insulating film 25. The gateelectrode 26 is equivalent to the word line WL shown in FIG. 9.

A contact plug 27 is formed on the diffusion region 23. The source lineSL is formed on the contact plug 27. A contact plug 28 is formed on thediffusion region 24. The lower electrode 29 is formed on the contactplug 28. The MTJ element 1 is provided on the lower electrode 29. Anupper electrode 30 is formed on the MTJ element 1. The bit line BL isprovided on the upper electrode 30. The space between the semiconductorsubstrate 21 and the bit line BL is filled with an interlayer insulatinglayer 31.

An example case where magnetoresistive elements according to one of thefirst through fourth embodiments and their modifications are used in aMRAM has been described so far. However, the magnetoresistive elementsaccording to the first through fourth embodiments and theirmodifications can also be used in any other devices utilizing the TMReffect.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein can be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein can be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. A magnetoresistive element comprising: a firstferromagnetic layer having an axis of easy magnetization in a directionperpendicular to a film plane, a magnetization direction of the firstferromagnetic layer being changeable; a second ferromagnetic layerhaving an axis of easy magnetization in a direction perpendicular to thefilm plane, a magnetization direction of the second ferromagnetic layerbeing fixed; a first nonmagnetic layer provided between the firstferromagnetic layer and the second ferromagnetic layer; and a thirdferromagnetic layer provided on an opposite side from the firstnonmagnetic layer relative to the second ferromagnetic layer, andincluding a plurality of ferromagnetic oscillators generating rotatingmagnetic fields of different oscillation frequencies from one another,each of the oscillators having a magnetization parallel to the filmplane, the rotating magnetic fields being generated by flowing a currentbetween the first ferromagnetic layer and the third ferromagnetic layerand at least one of the rotating magnetic fields being applied at amagnetization switching in the first ferromagnetic layer.
 2. The elementaccording to claim 1, further comprising a spin injection layerinjecting spin-polarized electrons into the third ferromagnetic layer,the spin injection layer being located between the second ferromagneticlayer and the third ferromagnetic layer.
 3. The element according toclaim 1, further comprising: a spin injection layer injectingspin-polarized electrons into the third ferromagnetic layer, the spininjection layer being located on an opposite side from the secondferromagnetic layer relative to the third ferromagnetic layer; and asecond nonmagnetic layer provided between the second ferromagnetic layerand the third ferromagnetic layer.
 4. The element according to claim 1,wherein the oscillation frequencies of the rotating magnetic fieldsgenerated from the oscillators of the third ferromagnetic layer are inthe range of 0.62f₀ to 1.50f₀, where a resonant frequency of the firstferromagnetic layer is f₀.
 5. The element according to claim 1, whereinthe third ferromagnetic layer includes a nonmagnetic matrix and aplurality of ferromagnetic grains surrounded by the nonmagnetic matrix,the ferromagnetic grains being the oscillators.
 6. The element accordingto claim 5, wherein the nonmagnetic matrix of the third ferromagneticlayer is an oxide containing at least one element selected from Al, Si,Ti, Mg, Ta, Zn, Fe, Co, and Ni, and the ferromagnetic grains contain atleast one element selected from Fe, Co, and Ni.
 7. The element accordingto claim 5, wherein the nonmagnetic matrix of the third ferromagneticlayer is a nonmagnetic metal material, and the ferromagnetic grainscontain at least one element selected from Fe, Co, and Ni.
 8. Theelement according to claim 1, wherein the third ferromagnetic layercomprises a ferromagnetic film and a nonmagnetic film, the ferromagneticfilm containing at least one element selected from Fe, Co, and Ni, thenonmagnetic film containing at least one element selected from Cu, Ag,Au, and Ru.
 9. The element according to claim 1, further comprising: afourth ferromagnetic layer provided between the second ferromagneticlayer and the third ferromagnetic layer, and having an axis of easymagnetization in a direction perpendicular to the film plane, amagnetization direction of the fourth ferromagnetic layer beingantiparallel to the magnetization direction of the second ferromagneticlayer; and a third nonmagnetic layer provided between the secondferromagnetic layer and the fourth ferromagnetic layer, wherein M_(S2)represents saturation magnetization of the second ferromagnetic layer,t₂ represents film thickness of the second ferromagnetic layer, M_(S4)represents saturation magnetization of the fourth ferromagnetic layer,t₄ represents film thickness of the fourth ferromagnetic layer, and therelationship, M_(S2)×t₂<M_(S4)×t₄, is satisfied.
 10. A magnetic randomaccess memory comprising: the magnetoresistance effect element accordingto claim 1; a first wire that is electrically connected to the firstferromagnetic layer; and a second wire that is electrically connected tothe third ferromagnetic layer.
 11. The memory according to claim 1,further comprising a selective transistor, one of a source and a drainof which being electrically connected to one of the first and thirdferromagnetic layers and the other of the source and the drain beingelectrically connected to one of the first and second wires.